1. Field of the Invention
The invention relates in general terms to a process for applying a coating to a biodegradable plastic substrate, and specifically to a process for applying a coating of this type in which a three-dimensional body, in particular a hollow body, is used as a plastic substrate and at least one barrier layer is deposited on the surface of the plastic substrate, and also relates to a composite material produced using the process.
The invention also relates to a composite material comprising a three-dimensional biodegradable plastic substrate and at least one coating having at least one barrier layer which is applied by chemical vapor deposition.
2. Relevant Art
Biodegradable plastics are becoming increasingly important on account of the growing problems of disposing of in particular packaging waste. Biodegradable plastics can be broken down into their basic constituents by the combined action of heat, moisture and micro organisms. They do not represent critical problems when disposed of by land fill, combustion, composting and recycling.
The degradation generally forms carbon dioxide, water and biomass, which can be returned to the nutrient cycle for example via the compost.
Biodegradable plastics can be produced using a very wide range of technologies, both from renewable resources, i.e. from raw materials of animal or vegetable origin which regrow, and from fossil resources. The biodegradable property is a consequence of the chemical composition of the plastic and not of the origin of raw material.
Nowadays, the following groups of materials based on raw materials which regrow are among those available: starch and starch blend, polylactides (PLA), cellulose, cellulose acetate (CA), polyhydroxybutyrate/valerate (PHB/PHV), polyhydroxyalkanoates, cellulose film, viscose, animal and vegetable raw materials and residue materials, such as gelatin. Certain polyesters and copolyesters, polycaprolactone (PCL), certain polyesteramides, polyester urethanes and polyvinyl alcohol (PVAL) are known biodegradable plastics based on fossil raw materials.
Some biodegradable plastics, such as polyhydroxybutyrate/valerate, can be produced and enriched directly in micro organisms or in plants. Others are generated from natural raw materials; one example is polylactide, which is based on lactic acid building blocks.
Polylactides have long been known for medical applications. They are considered to have good technological chances of economic production as future mass-produced plastics. Polylactides can be prepared using all conventional plastics processing methods and can therefore be used for a very wide range of products. Examples include thermoformed packaging materials, foamed shaped bodies, film packaging materials, woven fabrics, nonwovens, wound coverings and stretch blow molded hollow bodies.
Polyhydroxybutyrate has also long been known from the medical sector. In particular the specific properties of compatibility and degradability even in the human body are of interest for its use in implants, stitches, stitching and bandaging material and pharmaceutical capsules.
Further biodegradable plastics are being developed in particular for foodstuff packaging, but also generally in sectors in which biodegradability constitutes an advantage in use, for example in agriculture and forestry and in horticulture, as well as wherever the plastic is highly mixed with organic waste, so that its separation and conventional plastic recycling are not suitable. One example is thermoplastic starch.
The use of pure thermoplastic starch as a material is prevented by its extremely strong hydrophilicity. Therefore, to produce a thermoplastic material based on starch, it is necessary to modify the starch in order to make the material resistant to water and less sensitive to moisture. One possible way of doing this is to produce polymer mixtures with a water-resistant polymer.
The processing and use properties of the biodegradable plastics are often improved by additives, such as for example plasticizers or lubricants. Fillers are used to satisfy both optical and economic requirements. Therefore, finished products are often composite materials comprising different components. However, if the biodegradability or combustibility is to be retained, this factor must always be borne in mind when selecting the processes and/or materials.
To make it possible to dispense with the use of polymer blends when producing biodegradable plastics with satisfactory use properties, it is advantageous to provide plastic substrates with a barrier coating, in order to ensure a reduction in the permeation of gases and liquids, and furthermore to protect the plastics material from chemical attacks or UV radiation.
German Patent Document DE 199 544 03 A1 describes a multilayer biodegradable film with improved barrier properties with respect to oxygen and water vapor and also a process for producing it and its use. The improvement to the barrier action is achieved by providing the biodegradable film with a layer of a biologically harmless nonpolymeric material. The coating is in this case preferably applied by evaporation coating in vacuo, with the evaporation coating medium being evaporated thermally or by electron beams. The evaporated material can be activated before it actually comes into contact with the film surface by generating a plasma discharge in the material in vapor form using microwave radiation which has been introduced.
In practice, however, it has been found that known coating processes, when they are used to apply barrier layers to biodegradable plastic substrates, result in a considerable deterioration in the material properties of the plastic substrate. On account of the relatively low glass transition temperature of the biodegradable plastics, which is generally below 80° C., for example in the case of polylactides is 55° C., the material is softened on account of the higher temperatures used during the coating operation. Known processes fail in particular when they are used to coat shaped plastic bodies, since the shaped bodies readily lose their shape on account of the softening which occurs during coating.